1. Field of the Invention
This invention is concerned with a bidirectional optical module which transmits and receives light signals through a single optical fiber shared in common, and relates to an improvement of the bidirectional optical module, which is achievable of more cost reduction than the conventional.
2. Description of the Related Art
With expansion of optical fiber communication networks, a communication system which sends optical signals bidirectionally by the use of a single optical fiber is on the way to its introduction. In such bidirectional optical communication, a bidirectional optical module is used in order to transmit and receive light signals with different wavelengths.
The construction of a conventional bidirectional optical module is schematically shown in FIG. 10.
In the bidirectional optical module shown in FIG. 10, transmitting signal light with a wavelength λ1 (e.g., 1,310 nm) which has been emitted from a light-emitting device 101 is collected by a first coupling lens 102, passes through a wavelength splitting filter 103, and is thereafter made to enter an optical fiber 108 so as to be sent forward therethrough. On the other hand, receiving signal light with a wavelength λ2 (e.g., 1,490 nm) which has been sent backward through and made to emerge from the optical fiber 108 is reflected by the wavelength splitting filter 103, and is collected by a second coupling lens 110 to come to enter a light-receiving device 109, where signals are detected.
Where a distributed feedback laser (DFB laser), which has a high performance but tends to be affected by reflection return light (reflected return light), is used in the light-emitting device 101, an optical isolator constituted of a polarizer 104, a Faraday rotator 105, an analyzer 106 and a magnet 107 is disposed on an optical path extending between the first coupling lens 102 and the wavelength splitting filter 103. Where the light is one in the direction perpendicular to the polarization of light emitted from the light-emitting device 101, the polarizer 104 may be omitted because it hardly takes part in making oscillation unstable. An example of such a bidirectional optical module is disclosed in Japanese Patent No. 3062949.
Now, in the conventional bidirectional optical module having the above optical isolator, the optical isolator is disposed at a position where the beam diameter of the transmitting signal light emitted from the light-emitting device 101 is large (i.e., on the optical path extending between the first coupling lens 102 and the wavelength splitting filter 103). Hence, in respect of component parts (optical elements) such as polarizers and a Faraday rotator which constitute the optical isolator, those having a large aperture diameter (required to be about 1 mm at least) have had to be used, resulting unavoidably in a high cost.
As a conventional technique by which an optical isolator having a small aperture diameter can be set in, an optical module is known in which an optical isolator constituted of a pair of absorption type polarizers and a Faraday rotator which have integrally been set up is bonded at a position where the beam diameter becomes smallest (i.e., bonded to the optical fiber end) (see Japanese Patent Application Laid-open No. 2002-156554). However, the absorption type polarizers may inevitably function as polarizers not only for the transmitting signal light but also for the receiving signal light to come to loss factors, and hence it has been unable to use such an optical isolator in any bidirectional optical module.